1. Field of the Invention
The present invention relates to an electrolytic capacitor using a valvular metal such as aluminum, tantalum, or the like, to a method for producing the electrolytic capacitor, and, particularly, to an electrolytic capacitor using an conducting polymer as the cathode and its production method.
2. Description of the Prior Art
Customary electrolytic capacitors using a valvular metal e.g. aluminum, tantalum, are generally fabricated using a valvular metal porous body as the anode, an oxide of a valvular metal as the dielectric layer, and an liquid electrolyte or an inorganic solid electrolyte as the cathode. As the cathode, for example, an organic solvent including an organic acid or the like is used in an aluminum electrolytic capacitor and manganese dioxide or the like in a tantalum electrolytic capacitor. In addition to the above, lead portions connecting to the anode and the cathode and finally a casing are formed.
Excellent responsiveness at high frequencies has been demanded of electronic parts along with the digitization of circuits. It is therefore required for the electrolytic capacitor to be improved in the excellent responsiveness at high frequencies by a reduction in the resistance. In this situation, the use of a high conducting polymer as a cathode has been studied and developed.
The conducting polymers used for conventional electrolytic capacitors are produced by polymerizing corresponding monomers by an electrolytic oxidation polymerization method or a electrochemical oxidation polymerization method. In these conducting polymers, a dopant anion is coordinated to improve the electronic conductivity. The doping with the dopant is usually performed in a solution in which the dopant coexists with monomers when the monomer is polymerized.
As the dopant, various anions such as a chlorine ion, sulfuric ion, and organic acid ion are utilized. The organic acid-type anions having a molecular structure with a large steric hindrance to the free movement of the organic anion in a polymer are industrially used to restrain the dedoping of the dopant from the formed conducting polymer layer.
The electrolytic capacitor has the structural feature in which an oxide film is formed on the surface of a porous valvular metal and the surface of pores of the valvular metal and is used as a dielectric layer, and the valvular metal remaining inside the dielectric layer is used as a anode. When forming a cathode in the capacitor, it is therefore necessary to coat, with the conducting polymer, the entire surface of the valvular metal, specifically, extending from the open surface of the porous body to the surface of very intricate internal pores in an efficient manner. In the case of producing the conducting polymer layer by the electrochemically oxidizing polymerization method, though the conducting polymer is formed into a densified coating with a low resistance and high quality, only an insufficient coating of the conducting polymer is formed in the inside of the porous body, posing the problem that no original capacitance is created due to imperfect coating.
On the other hand, formation of the conducting polymer by the chemical oxidation polymerization method results in that a coating of the conducting polymer is entirely formed even on the surface of internal of the porous body with ease. The polymer, however, is basically formed as precipitates exhibiting poor density and adhesion to a dielectric layer as described in U.S. Pat. No. 4,697,001. Thus this method gives rise to the problem that no uniform conducting polymer layer with a sufficiently low resistance can be obtained in a stable manner.
An object of the present invention is to provide an electrolytic capacitor in which an conducting polymer layer can be efficiently formed even on the inner surfaces of micropores in a porous body constituting a anode to coat a dielectric layer perfectly with the polymer layer, thereby inducing the original capacitance in an oxidation polymerization method, and also to provide a method for producing such a electrolytic capacitor.
A further object of the present invention is to provide an electrolytic capacitor in which an conducting polymer layer is formed as a densified layer which is highly adhesive to a dielectric layer formed in a porous body and which can be used as a stable cathode with a low resistance and also to provide a method for producing the electrolytic capacitor.
The above objects of the present invention can be attained by the provision of an electrolytic capacitor comprising a anode formed of a valvular metal porous body, a dielectric layer formed of an oxide of the valvular metal, and a cathode formed of an conducting polymer layer, wherein the conducting polymer layer is an polymer layer containing an organic acid-type dopant and formed by processing the monomer by chemical oxidation polymerization in a solution and the conducting polymer layer is formed as a uniform coating on the surface of the dielectric layer in the porous body.
Illustrating in more detail, the conducting polymer layer forming the cathode in the electrolytic capacitor of the present invention comprises an conducting polymer layer which contains no organic acid-type dopant, and is formed as a lower film on the dielectric layer and an conducting polymer layer which contains an organic acid-type dopant as a upper film which is formed on the above conducting polymer layer.
The conducting polymer layer containing no organic acid-type dopant, which is formed by chemical oxidation polymerization method, is formed extending to the inside of pores of the porous body. This is one of the features of the chemical oxidation polymerization method. Because the organic acid-type dopant, which also acts as a surfactant, is not present, this conducting polymer layer has a excellent advantage in that it is highly adhesive to the dielectric layer and is produced in the form of film. Therefore the internal pores can be efficiently coated with the densified conducting polymer layer which is highly adhesive to the dielectric layer and extends from the surface of the porous body to the surface of internal pores.
On the other hand, generally the conducting polymer layer containing the organic acid-type dopant is to be produced as particulate precipitates, but it is experimentally confirmed that the conducting polymer layer containing the organic acid-type dopant grows as a densified film on the conducting polymer layer without containing an organic acid-type dopant which have formed. Accordingly, the above conducting polymer layer without containing an organic acid-type dopant is formed in advance whereby the conducting polymer layer containing organic acid-type dopant can be formed as a densified and filmed polymer layer and can have high conductivity.
An entire conducting polymer layer is formed as a layer, which may be increased in rate of covering the dielectric layer and decreased in resistance, which enables the capacitor have high capacitance and high responsiveness at high frequencies.
In order to provide the above electrolytic capacitor, when forming an conducting polymer layer by a chemical oxidation polymerization method, the production method of the electrolytic capacitor in the present invention comprises, in a pretreatment, a step of forming a film of an conducting polymer layer without containing an organic acid-type dopant on the surface of a dielectric layer formed of an oxide, and, as a primary treatment, a step of growing an conducting polymer layer containing an organic acid-type dopant on said lyer.
In the pretreatment of the above process, a porous body of a valvular metal formed with a dielectric layer is dipped into a solution containing a monomer and an oxidizing agent but not containing an organic acid-type dopant to form an conducting polymer layer without containing an organic acid-type dopant on surface on the dielectric layer by polymerizing the monomer.
Alternatively, in the pretreatment of the process, a valvular metal porous body formed with a dielectric layer is dipped alternately in a monomer solution and in a oxidizing agent solution neither of which contains an organic acid-type dopant to form an conducting polymer layer without containing an organic acid-type dopant on the surface of the dielectric layer by polymerize the monomer.
Next, in the primary treatment, the valvular metal porous body formed with the conducting polymer layer in the pretreatment may be dipped in a solution containing a monomer, an oxidizing agent, and an organic acid-type dopant to form an conducting polymer layer containing an organic acid-type dopant on the surface of the conducting polymer layer by polymerizing the monomer. In another primary treatment, the valvular metal porous body formed with the conducting polymer layer in the pretreatment may be dipped alternately in a monomer solution and oxidizing agent solution, at least either of which contains an organic acid-type dopant, to form an conducting polymer layer containing an organic acid-type dopant on the surface of the conducting polymer layer by a polymerization reaction.
In this method, first as the pretreatment chemical oxidation polymerization reaction of a monomer is performed in the solution include no organic acid-type dopant whereby the conducting polymer layer excluding the organic acid-type dopant can be formed on the surface of the dielectric layer as a densified film which is highly adhesive to the dielectric layer. Next, as the primary treatment, the polymerization reaction of the monomer is performed in a solution containing an organic acid-type dopant whereby the conducting polymer layer containing the organic acid-type dopant can grow as a film over the surface of the conducting polymer layer without containing an organic acid-type dopant. As a consequence, the conducting polymer layer formed as a film which is highly adhesive to the dielectric layer is used as the cathode which makes it possible to manufacture a capacitor achieving a high rate of the capacitance relative to the ideal capacitance to be calculable from the porous structure, and having lower resistance, and excellent responsiveness at high frequencies.